The astronomers who claim to have discovered the 10th planet in the solar system have another intriguing announcement: It has a moon.
While observing the new, so-called planet from Hawaii last month, a team of astronomers led by Michael Brown of the California Institute of Technology spotted a faint object trailing next to it. Because it was moving, astronomers ruled it was a moon and not a background star, which is not moving.
The moon discovery is important because it can help scientists determine the new planet's mass. In July, Brown announced the discovery of an icy, rocky object larger than Pluto in the Kuiper Belt, a disc of icy bodies beyond Neptune. Brown labeled the object a planet and nicknamed it Xena after the lead character in the former TV series "Xena: Warrior Princess." The moon was named Gabrielle, after Xena's faithful traveling assistant.
By determining the moon's distance and orbit around Xena, scientists can calculate how heavy Xena is. For example, the faster a moon goes around a planet, the more massive a planet is. Possessing a moon is not a criteria of planethood since Mercury and Venus are moonless planets. Brown said he expected to find a moon orbiting Xena because many Kuiper Belt objects are paired with moons.
The newly discovered moon is about 155 miles wide and 60 times fainter than Xena, the farthest-known object in the solar system. It is currently 9 billion miles away from the sun, or about three times Pluto's current distance from the sun.
Scientists believe Xena's moon was formed when Kuiper Belt objects collided with one another. The Earth's moon formed in a similar way when Earth crashed into an object the size of Mars.

宣称已在太阳系发现了第10颗行星的天文学家们又会有一个令人感兴趣的宣布：这颗行星有一颗卫星。当上个月在夏威夷观察到这颗所谓行星的同时，由加州技术学院的Michael Brown所带领的一队天文学家也观察到一个模糊的物体在这颗行星傍边游弋。由于这个物体是移动的，因此科学家们判定它是卫星，而未将其判定为不会移动的背景星。
这颗卫星的发现十分重要，因为它可以帮助科学家们确定这颗新行星的质量。七月，Brown宣布：在古比皮带发现了一个被冰覆盖的大于冥王星的岩石物体，一个远在海王星那一边的圆盘形覆盖着冰的天体。Brown将这个物体称为行星，并戏謔地用前段播放的电视联系剧《战士公主：西娜》中的女主角的名字称它为“西娜”。它的卫星则以“西娜”忠实的追随者“Gabrielle”来命名。
通过确定卫星环绕“西娜”的距离和轨道，科学家们可以计算出“西娜”到底有多大。例如：如果一颗卫星环绕行星的速度越快，这颗行星的质量则越大。拥有卫星并不是辨别行星的一种标准，因为水星和金星都是没有卫星的行星。Brown说：他预见到会发现卫星，因为许多位于古比皮带的天体都伴有卫星。
最新发现的卫星大约宽155哩，亮度比“西娜”弱 60倍，是目前所知在太阳系中最远的物体。它目前离太阳的距离是90亿哩，比冥王星目前离太阳的距离还远大约 3倍。
科学家们相信，“西娜”的卫星是由于古比皮带的物体与另一个物体相撞后而形成的。地球的卫星也是因同样的方式而形成，当时地球曾撞上了一个与火星大小相等的物体。

注：在天文学中，有些恒星因亮度太弱，观察起来非常模糊，故称
　　其为“背景星”＝background star